99% RH in concrete slab

[mwoody]

I have a project where 8 Wagner 4.0 RH probes were placed in a 40,000 square foot concrete slab on grade. The slab thickness is 6 inches with a vapor barrier on top of soil. The probes were placed at 40% of the depth of the slab. The slab was placed approximately 15 to 20 years ago. The HVAC has been running for over a month with the ambient temperature at 75 degrees and the humidity between 50 and 60%.

6 of the 8 probes read in the 70% to 80% range and the other two probes read 99%. The two 99% readings are on opposite sides of the warehouse. Any ideas on why the probes would read 99%?